Improvements in interdigital delay lines



Jan. 10,1956 0, OHLER ET AL 2,730,678

IMPROVEMENTS IN INTERDIGITAL DELAY LINES Filed Dec. 17, 1952 {IMPROVEMENTS IN INTERDIGITAL DELAY LINES oslcar. .Dohler and Bernard Epsztein, Paris, France, as-

signors to Compagnie Generale d e Telegraphic Sans Fri, a corporation of France 7 l lni ncanaiiue'cembn 11,1952, satin No. 326,552 Claims priority, application France December 29, 1951 r 4 Claims. (Cl, 333-31 .This invention relates to interdigital delay lines for use int'r'avelling wave tubes preferably of the crossed-fields type with propagation of energy in the direction opposite to the direction of electrontravel in the tube.

1' ma travelling wave tube of this type the frequency of oscillations is determined by the velocity of the electrons therein, andan electron beam forms a feedback path between .the output and the input circuits of the tube. Fer-operation of this tube it is necessary to provide a delay line the dispersion curve of which is abnormal, that'is", to say, a delay line propagating a wave having phase velocity 'of opposite sign to the group velocity.

""Su'ch delay lines are known, one type having. the above c aract e'ristic being knownas the interdigital type of line.

"The invention is ill'ustrated' in'and explained in connection with the accompanying schematic drawings, wherein Figures 1 and 2 are perspective views of two forms of known interdigital delay lines and Figures 3 to 5 are similar'views of three embodiments of delay lines according t6 thejinven'tion. Fig. l'of the accompanying drawings shows a known interdigital type of delay line constituted by two'rnetal combs 1 and 2, which are identical, and the fingers?) and 4, which are of rectangular form and are'isymmetrically interleaved. These lines offer great advantages, particularly thatof being able to be produced at little expense and with 'good geometrical accuracy. Nevertheless they have serious disadvantages, which will liejclear from the following analysis:

The delay rate of such lines, that is to say the ratio of 'phas'e-to-light velocity, is a function solely of the length of'the fingers and of the pitch of the line, that is to say of the distance between the axes of two adjacent fingers. Iffitfis desired to obtain high power from a tube of the type specified hereinabove, it is necessary to provide an electron'be'ani of great width and intensity. Such increase iiiwidth must however obviously go hand-in-hand with a c lative "modification of the characteristics of the line, for ifit "is'desired tdretaina given frequency of operation, itis'necessary to retain, for the phase velocity of the Wavebeing propagated in the line, a velocity substantially equal to the electron velocity. In other Words, it is necessary to; retaimfor theline the same length of fingers .and the same pitch. The only modification of the ime'r'is'ionsiofthe'line" which permits an increase of the width ofthe"'beairi,is anincrease of its width. It thus becomes necessaryto have a line ofthe shape shown in Fig. 2, wherethe same references designate the same palms in Fig. i, that ist'osay a line the fingers of which over-lap to only; a slight extent. As will be seenthe line shwh iiFFig'; Zhas a middle part where there are found fingers of the two combs, said part having a width a sub stantially equal to the width b of the two side parts where there are found fingers only of a single comb, whereas in the line shown in Fig. l the corresponding dimensions a was of the order of 10 times b.

' A delay line as shown in Fig. 2 is most unsatisfactory United States. Patent 0 ice if not actually useless in the type of tube specified above, for the following reasons: (11) Such a delay line involves troubles as regards lack of uniformity of the direct current electric field between the delay line and a negative electrode parallel thereto, which is usual in the tubes of the crossed-fields type. It will be observed that in the middle part the distance between two consecutive metal surfaces, which corresponds to the surface-to-surface distance from one finger 3 of a comb 1 to an adjacent finger 4 of the other comb 2, is small, while in the side parts there are, along the width b, relatively considerable distances between adjacent fingers 3 of the comb 1 and also between adjacent fingers 4 of the comb 2. If an electric field is established between the delay line and a negative electrode parallel thereto, the electric lines of force terminating on I the line will then be disturbed, and this will entail serious fined sense, that is to say in that region the exchange of energy is effected under suitable conditions for use in a backward-wave oscillator, whereas on the contrary, in the two side parts the high frequency energy is transportedas to its major part, by waves known as space harmonics, the dispersion of which is normal, that is to say the phase velocity is of the same sense as the group veloc-' ity, and under this condition in that region there cannot be an exchange of energy between electrons and waves when the tube is operating as backward-wave oscillator,

- since in this case the electron velocity is equal to the negative phase velocity and it cannot be equal at the same time to a positive phase velocity. Only the part corresponding to the middle part of the comb will therefore subsist as the useful part of the beam, and therefore the output of energy will be very low.

The present invention provides improved interdigital delay lines having slight interdigitated fingers but which is nevertheless free from these disadvantages.

According to the invention an interdigital delay line, of the above defined slightly interdigitated type, has fingers which are all identical, but which have, in the plane of the combs, stepwise or progressively decreasing width from their extremity which is secured to a comb, to their second extremity which is free.

In one embodiment of the invention the fingers of an interdigital line are formed from a comb body of afirst prismatic part of rectangular cross-section, and from a second, likewise prismatic part of substantially the same length and of rectangular cross-section, but having a subl stantially smaller area, these two parts having a common face perpendicular to the trajectory of the electrons and the two faces parallel to the trajectory of the electrons.

in alignment with one another. The fingers of the two combs are interleaved in such manner that the facesof the prisms of greater section are in line with one another.

' another," in such manner thatone face of the part of smaller section of the one is in line with one face of the part of greater section of the other.

In a third embodiment of the invention, the fingers are in the form of triangular prisms, the base of the prism being carried by the body of the comb, and are inter- 3 leaved in one another in such manner that the distance between two consecutive metal surfaces is substantially constant.

In Fig. 3, 1 and 2 are the bodies of the two combs, one of which carries the fingers 3 and the other the fingers 4. The fingers 3, instead of having a rectangular prismatic shape as shown in Fig. 1, have a first part 6 carried directly by the body of the comb, followed by a second part 7 likewise in the shape of the rectangular prism, substantially of the same length, and the width in the plane of the combs of which is substantially half the width of the part 6. These two prisms have a common face, and two faces which are respectively in line with one another.

In the shoulder thus formed on the fingers 3 are interleaved the fingers 4 of identical shape to that of the fingers 3, and the shoulder of which fits the shape of the shoulder of the fingers 3. Between these fingers a zig-zag shaped passage 8 of slight thickness is thus provided. Such a line behaves towards the direct current electric field prevailing in the interaction space practically as if there were no break in continuity between the fingers of the two combs; in other words, as if the line were composed of two bars 1 and 2 connected together by regularly spaced parallel bars. It is obvious that such construction involves little disturbance to the electric lines of force, the distance between two consecutive metallic surfaces, in the direction of the displacement of the electrons, being practically constant. Such a line, however, for which a is substantially equal to b, is able, like the line shown in Fig. 2, to be coupled with a beam of substantially increased width for the same delay rate, as compared to a line as shown in Fig. 1. In addition, experience shows that, contrarily to what happens in the case of the line illustrated in Fig. 2, the high frequency energy is transported, over practically the whole of the width of the line, by waves having abnormal dispersion and the same negative phase velocity. In other words, the electron-wave exchange of energy is effected over the entire width of the line under suitable conditions for use in a backwardwave oscillator, which has the result that the output is high, which, as has been seen, is not the case for the line illustrated in Fig. 2.

Fig. 4 illustrates another delay line according to the invention. The same references designate the same parts as in Fig. 3. The fingers 3, as in the preceding case, have two parts 6 and 7 substantially of the same length, and likewise in the shape of a rectangular prism, the width in the plane of the combs of the second part being substantially smaller than the width of the first part. But in this case the two faces of the two prisms perpendicular to the electronic trajectories are not in line with one another, but are stepped on both sides. Two shoulders are thus formed. In one of the two is accommodated an identical shoulder of a finger of the other comb, constructed in identical fashion so that one of the faces perpendicular to the trajectory of the electrons of the second part of this second finger is in line with one of the faces of the first part of the first finger, a zig-zag shaped passage 8 of slight thickness remaining between the two fingers. This delay line has substantially the same advantages as that illustrated in Fig. 3, and behaves towards the direct current electric field of the interaction space substantially in the same manner.

In the delay line illustrated in Fig. the fingers have the shape of triangular prisms, the base of which rests on the body of the corresponding comb.

In this figure the same references again designate the same parts as in the preceding figures. In Fig. 5 it can be seen that each finger 3 of the comb 1 forms a pair with a finger 4 of the comb 2, said fingers being separated by an oblique passage 9 of constant thickness, whilst the distance between two consecutive pairs 3, 4 is subtantially constant over the entire width of the line. In Fig. 5 also the dimension a is substantially equal to dimension b.

Lines according to this invention retain the advantages of not causing disturbance to the lines of force of the electric field in the interaction space while energy is transported by a wave of abnormal dispersion over the entire width of the line.

The invention is not restricted to the arrangements described, but is susceptible of modification within the ambit of the appended claims.

We claim:

1. A delay line for a travelling wave tube, comprising two combs, each having a series of parallel fingers having a first extremity secured thereto and a second free extremity, the fingers of both combs being reciprocally interdigitated, the distance between said free extremity of each finger of one of said combs and the other comb being of the order of one-third of the total distance between said combs, said fingers having, in the plane of said combs, decreasing width from said first extremity to said second extremity.

2. A line as claimed in claim 1, in which the fingers are formed, starting from the body of the comb, of a first prismatic part of rectangular cross-section and a second, likewise prismatic part substantially of the same length as the first part but having a substantially smaller rectangular cross-section, one side face of the fingers, perpendicular to the axis of the line, being common to these two parts, and the side faces parallel to the axis of the line being in the same plane for the fingers of the two combs, said second prismatic parts of the opposite fingers being reciprocally interleaved.

3. A line as claimed in claim 1, in which the fingers are formed, starting from the body of the comb, of a first prismatic part of rectangular cross-section and a second, likewise prismatic part substantially of the same length as the first part but having a substantially smaller rectangular cross-section, these two parts having the same medium plane, and the side faces of the fingers parallel to the axis of the line being in the same plane for the fingers of the two combs, said second prismatic parts of the opposite fingers being reciprocally interleaved in such manner that one face, perpendicular to the axis of the line, of each second part is in the same plane as a face, perpendicular to the axis of the line, of said first part of the opposite finger.

4. A line as claimed in claim 1, in which the fingers are in the form of triangular prisms each having one face carried by the comb, a second face perpendicular to the axis of the line, and a third face inclined in relation to said axis, said fingers being interleaved in such manner that said inclined faces are substantially parallel and bound to a passage of substantially constant width.

References Cited in the file of this patent UNITED STATES PATENTS Everhart Dec. 5, 1950 Cohn Nov. 11, 1952 OTHER REFERENCES 

